This invention generally relates to water jet-propelled boats or watercraft having an inboard motor and an outboard water jet propulsion unit. In particular, the invention relates to methods for suppressing noise and damping vibrations produced by the outboard water jet propulsion unit.
It is known to propel a boat or other watercraft using a water jet apparatus mounted to the hull, with the powerhead being placed inside (inboard) the hull. The driven shaft of the water jet apparatus is coupled to the drive shaft of the inboard motor. The impeller is mounted on the driven shaft and housed in a jet propulsion duct or water tunnel or housing.
To facilitate use of water jet-propelled boats in shallow water, it is known to mount the water jet propulsion assembly at an elevation such that the unit does not project below the bottom of the boat hull. This can be accomplished, for example, by installing a duct in the stern of the boat, the duct being arranged to connect one or more inlet holes formed in the bottom of the hull with an outlet hole formed in the transom. The water jet propulsion assembly is then installed outside the hull in a position such that its inlet is in flow communication with the duct outlet at the transom. Alternatively, the water jet impeller can be installed inside the duct built into the hull.
It is further known to integrally form an inlet ramp or tunnel in the stern portion of the bottom of a hull. The inlet ramp comprises a pair of opposing side walls which increase in height continuous-ly from a starting point on the hull bottom to the respective points where the side walls join the hull transom. The top edges of the opposing side walls are connected by a ramp ceiling which curves continuously upward. The side walls and ceiling form part of the hull bottom and define an inlet channel. Optionally, the junctures connecting the side walls to the ceiling may be formed as rounded, as opposed to sharp, corners. A mounting adapter in the form of a flanged ring having a rounded leading lower lip is mounted to the rear face of the hull transom. The bottom edges of the inlet ramp and the forward tip of the lower lip define an inlet opening for entry of ambient water into the inlet channel formed by the inlet ramp.
The mounting adapter is mounted to the transom by fasteners. The water jet propulsion assembly is in turn mounted to the mounting adapter in cantilever fashion in a well-known manner. The outlet of a discharge nozzle of the water jet propulsion assembly is in flow communication with the inlet opening in the hull bottom via the hull inlet ramp, the mounting adapter, and one or more housings of the water jet propulsion assembly itself (e.g., an impeller housing and a stator housing). All of these components, communicating with each other in series, form a duct having a channel with an inlet and an outlet. Rotation of an impeller, driven by an inboard. motor, produces flow through the duct in a well-known manner.
In accordance with the latter design, the water jet propulsion system, is mounted to the hull by means of a mounting adapter, which is typically made of metal, e.g., aluminum alloy. Acoustic waves (i.e., noise) generated during operation of the water jet propulsion system are transmitted to the mounting adapter, which in turn provides a path for acoustic waves to impinge on the hull and even enter the hull via penetrations, e.g., bolts which fasten the mounting adapter to the transom. This increases the levels of noise which the boat passengers or watercraft riders are exposed to. Similarly, if the mounting adapter is bolted directly to the hull, and the water jet propulsion unit is in turn bolted to the mounting adapter, then vibrations produced by the rotating impeller will cause the duct of the propulsion to vibrate, which in turn causes the adapter to vibrate, which in turn causes the hull to vibrate, and so forth.
There is a need for means and techniques for reducing the levels of noise and vibration to which boat passengers and watercraft riders are exposed. The means for reducing noise and vibrations reaching the hull should also act as a seal preventing ingress of water into the hull and ingress of air into the duct of the water jet propulsion system. Such means for reducing noise and vibration levels should be easy to install and relatively inexpensive to manufacture.
The present invention is directed to a jet-propelled boat comprising a water jet propulsion unit which is mounted to the boat hull by means of vibration isolators. In accordance with the preferred embodiment, vibration isolation material is inserted between the water jet propulsion system and the boat hull at every point where, in the absence of the vibration isolation material, the water jet propulsion system would contact the boat hull.
A jet-propelled boat in accordance with a preferred embodiment comprises a hull, a mounting adapter mounted to the hull, a water jet propulsion system mounted to the mounting adapter, and seals arranged at the interface of the hull and the mounting adapter. The seals are made of a vibration isolation material such as rubber. The mounting adapter comprises a mounting flange which opposes a portion of the hull transom. A first seal comprises a generally planar layer of flexible vibration isolation material arranged between and in contact with the transom and the mounting flange. The mounting adapter also comprises an inlet housing which sits inside an inlet ramp formed in the hull. [A mounting adapter comprising an inlet housing and a mounting flange will occasionally be referred to herein as an xe2x80x9cinlet adapterxe2x80x9d.] The inlet ramp has a recess for receiving a leading portion of the inlet housing. A second seal comprises a bead of flexible vibration isolation material placed in the recess. The bead sits between and in contact with the inlet ramp and the leading portion of the inlet housing. The first and second seals are preferably formed as a single molded piece, with the seals being connected by strips formed during molding.
The present invention is further directed to a compound seal for isolating a boat hull from vibrations produced during operation of a water jet propulsion unit mounted to the hull. A compound seal in accordance with a preferred embodiment comprises: a generally planar layer of flexible material having a peripheral edge forming a generally U-shaped opening; a generally U-shaped bead made of flexible material, the shapes and sizes of the opening and bead being generally similar, and means for connecting the bead to the layer. The bead lies generally parallel to the layer and is separated therefrom. The bead is generally aligned with and overlies the peripheral edge of the layer when viewed from in front of the bead along a line of sight generally perpendicular to the layer. Preferably the bead, layer and connecting means are made of the same flexible vibration isolation material, e.g., rubber, and are parts of a single molded piece.
By placing vibration isolation material at the interface of the mounting adapter and the hull, the hull can be substantially isolated from the vibrations produced during operation of the water jet propulsion unit. In addition, since typical vibration isolation material has high acoustic impedance, a substantial portion of the acoustic waves are absorbed or reflected, rather than being transmitted from the water jet propulsion unit to the unit. This reduces noise levels inside the boat. Furthermore, because the preferred vibration isolation material is flexible, when compressed between the mounting adapter and the hull, the isolation material acts as a water seal which prevents the ingress of water into the hull, e.g., via hull penetrations.